The present invention relates to a communicating method for a data communications apparatus which is capable of performing full-duplex communication over a two-wire circuit.
As well known in the art, communications systems may generally be classified into two types, i.e., a full duplex communications type which allows data to be transmitted simultaneously in both directions, and a semiduplex communications type which allows data transmission to occur simultaneously in one direction. Usually, a full-duplex communications type system is implemented by a four-wire circuit because it requires two data transmission channels, while a semiduplex communications type system require is implemented by a two-wire circuit because it needs only a single data transmission channel. Hence, once a particular kind of circuit is laid at the stage of installation of terminal units, the communications system cannot be changed any longer. It is true that a transmission system has been put to practical use which frequency division multiplexes the transmission band into a data transmision channel (forward channel) and a control signal transmission channel (backward channel) and, using a two-wire circuit, allows one terminal to transmit control signals representative of data receipt conditions and others to the the other terminal while the latter is transmitting data, or information, to the former. Yet, this kind of transmission system cannot be regarded as a perfect implementation for full-duplex communication.
Meanwhile, with the advent of advanced echo cancelling techniques, there has recently been realized a modem capable of securing two independent data transmission channels by the use of a two-wire circuit, e.g. a modem prescribed by CCITT (Consultive Committee of International Telegraph and Telephone) Recommendation V.26 ter. A modem with such a capability has broken new ground for full-duplex communication using a two-wire circuit. Outstanding advantages attainable with full-duplex communication over a two-wire circuit are remarkable cost-cutting and enhancement of transmission efficiency.
However, if full-duplex communication is performed between remote stations under poor circuit conditions due to deterioration of circuit characteristics, data cannot be exchanged in an adequate manner and the transmission is simply wasted. In addition, where the communication mode set up is the full-duplex mode, it is impossible to replace it with a semiduplex mode and, so, data cannot be transmitted until the circuit conditions are restored to normal.